Salts that have an organic, inorganic, or organometallic cation and a non-nucleophilic counteranion are widely used in industry as photoinitiators for cationic addition polymerization reactions. Such salts are also known to be useful as latent Brönsted- or Lewis-acid catalysts for step-growth (or condensation) polymerization, depolymerization, and unblocking of functionalized polymers.
Common photoinitiator salts include onium salts such as the diaryliodonium, triarylsulfonium, and (cyclopentadienyl)(arene)iron+ salts of the anions PF6−, AsF6−, and SbF6−. For such salts, it is known that the identity of the anion can significantly affect the rate of polymerization that can be achieved. The hexafluoroantimonate anion, SbF6−, for example, is associated with relatively fast cures and is often used in industrial applications.
Although onium salts are typically the initiators of choice for photocationic addition polymerization reactions, they can contain toxic elements and exhibit poor solubility in many organic solvents. The onium salts and their byproducts also tend to persist after polymerization, thereby imparting a strong acidic character to the resulting polymerized material.
Furthermore, with highly fluorinated anions, free hydrofluoric acid can be formed in the polymerized material through thermal degradation or hydrolysis of the anion. This can lead to thermal instability of the polymerized material or corrosion of substrates on which the polymerized material is placed.
One solution to this problem has been to add a base to the starting polymerizable composition as a neutralizing agent for the acid. However, bases can reduce the rate of polymerization and can leave unwanted byproducts of the neutralization reaction in the polymerized material. Another approach has been to add a compound capable of forming a water-insoluble or barely water-soluble compound, which, after the polymerization reaction, enters into a neutralization reaction with the acid generated by the cationic polymerization initiator at the time of polymerization.
It has been found that salts comprising anions having three highly fluorinated alkylsulfonyl, fluorinated arylsulfonyl, or perfluoroalkylsulfonyl groups, (or combinations thereof) exhibit improved solubility in organic media (relative to, for example, PF6− and SbF6− analogs) and have relatively strong catalytic activity. For example, cationic photoinitiators comprising a tris-(trifluoromethanesulfonyl)methide (“methide”) anion provide polymerization rates in cationic addition polymerization reactions comparable to those provided by photoinitiators that contain SbF6−.
In addition, the methide salts are stable, non-nucleophilic, do not readily hydrolyze to release fluoride ions that can be corrosive, and do not contain highly toxic elements such as arsenic and antimony. Such lack of corrosivity is important, as cationically polymerized materials (for example, conductive adhesives) are used in the electronics field, often in contact with multiple metal surfaces such as copper, tin, and indium-tin oxide. Epoxy compositions containing SbF6− anion tend to corrode indium-tin oxide surfaces whereas compositions containing methide anion show significantly less corrosion.
Although the methide salts seem to be direct replacements for SbF6−-containing initiators in cationic addition polymerization reaction systems as far as polymerization rate is concerned, it has been found that methides impart significantly reduced thermal stability to a resulting polymerized material. For example, epoxy formulations that have been photopolymerized using methide photoinitiators show significant decomposition at elevated temperatures compared with similarly formulated systems that have been photopolymerized using SbF6− salts.